Dramatic effect of pore size reduction on the dynamics of hydrogen adsorbed in metal–organic materials
Georgiev, Peter A.
Embs, Jan Peter
Zaworotko, Michael J.
KAUST Grant NumberFIC/2010/06
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AbstractThe effects of pore size reduction on the dynamics of hydrogen sorption in metal-organic materials (MOMs) were elucidated by studying SIFSIX-2-Cu and its doubly interpenetrated polymorph SIFSIX-2-Cu-i by means of sorption, inelastic neutron scattering (INS), and computational modeling. SIFSIX-2-Cu-i exhibits much smaller pore sizes, which possess high H2 sorption affinity at low loadings. Experimental H2 sorption measurements revealed that the isosteric heat of adsorption (Qst) for H2 in SIFSIX-2-Cu-i is nearly two times higher than that for SIFSIX-2-Cu (8.6 vs. 4.6 kJ mol-1). The INS spectrum for H2 in SIFSIX-2-Cu-i is rather unique for a porous material, as only one broad peak appears at low energies near 6 meV, which simply increases in intensity with loading until the pores are filled. The value for this rotational transition is lower than that in most neutral metal-organic frameworks (MOFs), including those with open Cu sites (8-9 meV), which is indicative of a higher barrier to rotation and stronger interaction in the channels of SIFSIX-2-Cu-i than the open Cu sites in MOFs. Simulations of H2 sorption in SIFSIX-2-Cu-i revealed two hydrogen sorption sites in the MOM: direct interaction with the equatorial fluorine atom (site 1) and between two equatorial fluorine atoms on opposite walls (site 2). The calculated rotational energy levels and rotational barriers for the two sites in SIFSIX-2-Cu-i are in good agreement with INS data. Furthermore, the rotational barriers and binding energies for site 2 are slightly higher than that for site 1, which is consistent with INS results. The lowest calculated transition for the primary site in SIFSIX-2-Cu is also in good agreement with INS data. In addition, this transition in the non-interpenetrating material is higher than any of the sites in SIFSIX-2-Cu-i, which indicates a significantly weaker interaction with the host as a result of the larger pore size. This journal is © the Partner Organisations 2014.
CitationNugent P, Pham T, McLaughlin K, Georgiev PA, Lohstroh W, et al. (2014) Dramatic effect of pore size reduction on the dynamics of hydrogen adsorbed in metal–organic materials. Journal of Materials Chemistry A 2: 13884. Available: http://dx.doi.org/10.1039/c4ta02171a.
SponsorsB.S. acknowledges the National Science Foundation (Award no. CHE-1152362) and the computational resources that were made available by an XSEDE Grant (no. TG-DMR090028). The authors also thank Dr Jonathan L. Belof for discussions on the two-dimensional quantum rotation calculations. This publication is also based on work supported by Award no. FIC/2010/06, made by the King Abdullah University of Science and Technology (KAUST). This work is based in part upon experiments performed at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany, and on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. This research project has also been supported by the European Commission under the 7th Framework Programme through the 'Research Infrastructures action of the 'Capacities' Programme, NMI3-II Grant number 283883. One of us (P.A.G) gratefully acknowledges financial support by the Project BeyondEverest under EU programme REGPOT-2011-1.
PublisherRoyal Society of Chemistry (RSC)
JournalJournal of Materials Chemistry A